Here is the lead sentence from a CNN piece on vaccine boosters:

Even though the biopharmaceutical company Pfizer has announced that it might be time to consider giving a third dose of its coronavirus vaccine to people, many doctors and public health officials argue that it's more beneficial to get shots into the arms of the unvaccinated right now than to boost those who are already fully vaccinated.

Delaying the 3rd dose to get out more 2nd doses is a perfectly reasonable position. What's interesting is that today delaying the 3rd dose is conventional wisdom and yet this is exactly the same argument that I made for delaying the 2nd dose, i.e. first doses first (FDF), back in December of 2020. At that time, however, the argument was controversial. My point, isn't that FDF has won the argument. My point is that what we are seeing, then and now, is status quo bias.

In December, status quo bias meant that people wanted to find a reason to stick with the status quo, i.e. 2 doses, and so they argued that delaying the second dose was "risky." Today, people still want to stick with the status quo and so they argue that third doses are "risky", i.e. delaying the third dose is now the less risky idea. The argument--it's smart to protect more people with fewer doses--hasn't changed but, without even realizing it, people are now making the argument that they once denied.

The logic hasn't convinced people but previously the logic opposed the status quo and now it supports the status quo so what was once denied is now accepted. What was in December the riskier choice now becomes the safer choice. With motivated reasoning, when the motivation changes so does the reasoning.

Hat tip: Iamamish

Addendum: People will respond in the comments but actually the situations are different. Yes, we might have magically hit on the optimal regimen of 2 doses, just like Goldilocks. But in fact that's not what happened. If we had started with a 3-dose regimen, delaying the third dose would have seemed just as "risky" as delaying the first dose in a 2-dose regimen.